ramips: mt7621: add NETGEAR R7200 as DEVICE_ALT1

Netgear R7200 is another clone of Netgear R6700v2, introduced in:
6e80df5 ("ramips: add support for NETGEAR R6700v2/AC2400")

Reported-by: Joel Pinsker, github user @joelp64
Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>

target/linux/ramips/image/mt7621.mk

@@ -624,6 +624,10 @@ define Device/netgear_r6700-v2
   DEVICE_VARIANT := v2
   DEVICE_ALT0_VENDOR := NETGEAR
   DEVICE_ALT0_MODEL := Nighthawk AC2400
+  DEVICE_ALT0_VARIANT := v1
+  DEVICE_ALT1_VENDOR := NETGEAR
+  DEVICE_ALT1_MODEL := R7200
+  DEVICE_ALT1_VARIANT := v1
   SERCOMM_HWNAME := R6950
   SERCOMM_HWID := BZV
   SERCOMM_HWVER := A001